Nutrient limitation suppresses the temperature dependence of phytoplankton metabolic rates

Emilio Marañón,María P Lorenzo,Pedro Cermeño,Beatriz Mouriño-Carballido

doi:10.1038/s41396-018-0105-1

Abstract

Climate warming has the potential to alter ecosystem function through temperature-dependent changes in individual metabolic rates. The temperature sensitivity of phytoplankton metabolism is especially relevant, since these microorganisms sustain marine food webs and are major drivers of biogeochemical cycling. Phytoplankton metabolic rates increase with temperature when nutrients are abundant, but it is unknown if the same pattern applies under nutrient-limited growth conditions, which prevail over most of the ocean. Here we use continuous cultures of three cosmopolitan and biogeochemically relevant species (Synechococcus sp., Skeletonema costatum and Emiliania huxleyi) to determine the temperature dependence (activation energy, Ea) of metabolism under different degrees of nitrogen (N) limitation. We show that both CO2 fixation and respiration rates increase with N supply but are largely insensitive to temperature. Ea of photosynthesis (0.11 ± 0.06 eV, mean ± SE) and respiration (0.04 ± 0.17 eV) under N-limited growth is significantly smaller than Ea of growth rate under nutrient-replete conditions (0.77 ± 0.06 eV). The reduced temperature dependence of metabolic rates under nutrient limitation can be explained in terms of enzyme kinetics, because both maximum reaction rates and half-saturation constants increase with temperature. Our results suggest that the direct, stimulating effect of rising temperatures upon phytoplankton metabolic rates will be circumscribed to ecosystems with high-nutrient availability.

Highlights

Temperature is a master variable that controls biological activity through its effect on metabolic rates [1,2,3]
Our results show that nutrient limitation suppresses the temperature dependence of metabolic rates, which means that the direct response of phytoplankton primary production to increasing ocean temperatures will differ fundamentally among ecosystems with different nutrient availability
The C: N elemental ratio of particulate organic matter responded to the degree of nutrient limitation, taking lower values at faster dilution rates in both S. costatum and E. huxleyi (Fig. S6B,D)

Summary

Introduction

Temperature is a master variable that controls biological activity through its effect on metabolic rates [1,2,3]. Within the temperature range of normal activity, metabolic rates increase with temperature according to the Boltzman–Arrhenius function:. Q10 1⁄4 ðR2=R1Þ10=ðT2ÀT1Þ ð2Þ where R2 and R1 are the rates measured at temperatures T2 and T1, respectively. Warmer temperatures will likely cause, in low-latitude, open-ocean regions, a reduction in phytoplankton productivity, as a result of enhanced thermal stratification and lower nutrient supply from sub-surface waters [9, 10]. This indirect effect could be counterbalanced by the direct, stimulating effect of increasing temperature upon phytoplankton growth [11,12,13,14]

Methods

Results

Conclusion